Polymer emulsions or polymer latices prepared by conventional emulsion homo- or co-polymerization of vinyl acetate using polyvinyl alcohol as protective colloid are widely used for adhesives. There are several application fields for such emulsions such as in wood glues, paper and packaging adhesives (e.g. case sealing, book binding, bag making, card board lamination), parquet adhesives and others. Most importantly these latices are used in paper and packaging or in woodworking adhesive formulations or wood glues.
All these adhesives and especially woodworking adhesives are optimized with respect to several properties such as water resistance of the bond, heat resistance of the bond and creep of the bond. Further, before application several dispersion properties must be met. These are compatibility with formulation aids such as film forming agents, fillers, metal salts; rheological properties; storage stability; the setting speed and so on.
Vinyl acetate (VAc) based polymer latices for paper and packaging have to meet certain requirements regarding setting speed, penetration into the paper, open time, cohesion of the bond. On the other hand they should possess good re-dispersability of a film to provide good machine cleaning properties.
VAc based polymer latices for woodworking adhesives having good water resistance typically use some functional post-crosslinking co-monomer in polymerization and the formulation with a catalyst afer polymerization. Amongst the post-crosslinkers N-alkylol derivatives of an amide of an xcex1,xcex2-ethylenically unsaturated carboxylic acid, such as N-methylol acrylamide (NMA) which is preferred for latex wood glues are known. For example, U.S. Pat. No. 5,439,960 and U.S. Pat. No. 5,391,608 describe the use of 0.1 to 6% of NMA based on total monomer content Likewise European Patent Application EP-A-0,561,221 describes the use of 2 to 10% NMA.
As the catalyst for formulation of the above woodworking adhesives having good water resistance water-soluble metal salts of monobasic acids such as aluminumm(III), chromium and/or zirconium(IV) salts, especially aluminum nitrate, chromium chloride or zirconium oxychloride are used (see for example DE-26,20,738, EP-A-0,433,957, EP-A-0,501,174, EP-A-0,686,683 and U.S. Pat. No. 5,434,216).
Another approach to improve water resistance of wood glues is for example disclosed in EP-A-0,433,957, EP-A-0,623,661 and JP-06,322,337 which documents relate to use of more hydrophobic co-monomers such as vinyl pivalate, diisopropyl maleate, diisopropyl fumarate, vinyl chloride or versatic acid vinyl esters for this purpose.
Adhesive performance of for example a wood glue is typically controlled by the amount of polyvinyl alcohol, i.e. the ratio of polyvinyl alcohol to monomer, the polyvinyl alcohol molecular weight and its degree of hydrolysis as for example disclosed in U.S. Pat. No. 5,434,216. In addition, functionalized polyvinyl alcohols, e.g. acetoacetylated polyvinyl alcohols or itaconic acid-modified polyvinyl alcohols, or ethylene modified PVOH may be used.
The choice of the polyvinyl alcohol (PVOH) determines other properties of the vinyl acetate latex and thus the adhesive obtained, especially its rheological properties such as viscosity and shear thinning, or the setting speed. Rheology mainly depends on the molecular weight distribution and degree of hydrolysis of the polyvinyl alcohol used, but can be changed by altering product recipe. For example, EP-A-0,686,683 discloses altering the rheological properties by altering the formulation with some thickening agent, thereby changing the continuous phase viscosity, at the expense of an altered recipe.
EP 00 114 097.9, assigned to Air Products Polymers LP, discloses adjustment of rheological properties and expecially reduction of shear thinning of a latex emulsion by increasing particle size of the polymer without changing overall composition. More in detail, the document teaches increasing particle size by carrying out emulsion polymerization of vinyl acetate and optional co-monomers in presence of a pre-crosslinking monomer selected from ethylenically unsaturated monomers such as triallyl cyanurate and allyl methacrylate. The document is silent on the possibility of increasing shear thinning factor.
With the term shear thinning the phenomenon of a viscosity drop with increased shear rate is addressed. Such decrease of viscosity is considered reversible if viscosity reaches the original value when the shear rate is decreased to its original value.
Shear thinning can be quantified by the so called xe2x80x9cshear thinning factorxe2x80x9d (SF) which is obtained as the ratio of viscosity measured by Brookfield viscosimeter at 1 rpm, Bf(1), and at 10 rpm, Bf(10):
SF=((Bf1/Bf10)xe2x88x921).
A shear thinning factor below zero (SF less than 0) indicates shear thickening, a shear thinning factor of zero (SF=0) indicates Newtonian behavior and a shear thinning factor above zero (SF greater than 0) stands for shear thinning behavior. Shear thinning factorsxe2x89xa00 reflect properties of emulsions with high solids content as is typically the case for VAc based latices. Non-Newtonian behavior is in general no problem at lower solids contents (e.g. below 40%).
It is an object of the present invention to find a possibility for independent control of the shear thinning factor which in turn allows to precisely and intentionally adapt the emulsion or latex for certain applications thereby improving performance of for example a paper and packaging adhesive or of a woodworking adhesive or wood glue.
In several cases it would be especially useful and desirable to control shear thinning i.e. the shear thinning factor and to provide the desired extend of it. More specifically, it would in the first place be desirable to avoid any shear thickening i.e. to be in the position to shift the shear thinning factor from negative to positive values (SF greater than 0). Performance requirements for a paper and packaging adhesive are, however, typically a SF within the range of 0.1 less than SFxe2x89xa61.5 dependent on the specific application, whereas performance requirements for a wood glue are typically a SF in the range of 0.1 less than SFxe2x89xa60.6. In any case shear thickening (SF less than 0) is preferably to be avoided.
Additionally, the requirements for setting speed and cohesion must be fulfilled: In the case of a paper and packaging adhesive setting speed measured as AZG in seconds should be: less than 4 s, whereas cohesion measured at 70xc2x0 C. should bexe2x89xa64 N/mm2.
In the case of a woodworking adhesive (wood glue) the setting speed measured as bond strength after 2.5 minutes gluing time should be greater than 1.5 N/mm2. Cohesion of the wood bond measured as bond strength at 80xc2x0 C. should be greater than 6 N/mm2.
Other useful adhesive performance parameters created e.g. by the level of co-monomer such as NMA, and/or by the amount of PVOH and/or by its molecular weight and hydrolysis distribution, should remain more or less unchanged.
It is thus another object of the invention to provide a wood glue which meets the above requirements and has improved water resistance.
The above objects are solved and the drawbacks of the prior art are overcome according to a first aspect of the invention by a vinyl acetate based polymer latex composition obtained by emulsion polymerization of:
(a) polyvinyl alcohol,
(b) a monomer mixture comprising vinyl acetate, and optionally (d) one or more additional co-monomer(s),
in presence of (c) 0.0001 to 0.05 wt. % (1 to 500 ppm) of a chain transfer agent (CTA), based on total monomer weight, said vinyl acetate based polymer latex composition having a weight average particle sizexe2x89xa7400 nm and a higher shear thinning factor than the vinyl acetate based polymer latex composition obtained in the absence of the chain transfer agent.
According to a first embodiment the emulsion polymerization is carried out in presence of 0.001 to 0.05 wt. % of a chain transfer agent selected from aldehydes, especially C2-5-aldehydes and other CTAs having a similar chain transfer constant with regard to VAc as the monomer.
According to a second embodiment the emulsion polymerization is carried out in presence of 0.0001 to 0.01 wt. % of a chain transfer agent selected from thiols, especially C2-5-thiols, mercapto acids, especially C2-5-mercapto acids and other CTAs having a similar chain transfer constant with regard to VAc as the monomer.
The latex of the invention preferably has a shear thinning factor of about 1,2 to 3,5 times the shear thinning factor of a latex composition obtained in absence of the chain transfer agent or wherein the shear thinning factor is shifted from negative values to positive values as compared to the shear thinning factor of a latex composition obtained in absence of the chain transfer agent.
Preferably, the latex weight average particle size is about 60 to 95% the weight average particle size of a latex composition obtained in the absence of the chain transfer agent.
Preferably, the chain transfer agent is selected from the group consisting of aldehydes, preferably C2-5-aldehydes, thiols, preferably C2-5-thiols, mercapto acids, preferably C2-5-mercaptoacids, and mixtures thereof. More preferably the chain transfer agent is selected from the group consisting of acetaldehyde, croton aldehyde, propionic aldehyde, 2-mercapto propionic acid, 3-mercapto propionic acid, and mixtures thereof.
The amount of vinyl acetate is preferably in the range of 100 wt. % to 60 wt. %, based on total monomer weight.
According to an especially preferred embodiment the vinyl acetate based polymer latex composition is a copolymer of vinyl acetate/N-methylol acrylamide, optionally further including versa tic acid vinyl ester having 9 to 13 carbon atoms from the carbonic acid group. The chain transfer agent is then preferably a mercaptopropionic acid.
According to a second aspect the present invention relates to a process of manufacture of a vinyl acetate based polymer latex composition having a weight average particle sizexe2x89xa7400 nm, including emulsion polymerization of vinyl acetate in the presence of polyvinyl alcohol, optionally together with one or more additional co-monomer(s), wherein the emulsion polymerization is carried out in the presence of 0.0001 to 0.05 wt. % of a chain transfer agent, based on total monomer weight. Preferably the chain transfer agents and the amounts used are as defined above.
According to a third aspect the present invention relates to an adhesive, comprising the vinyl acetate based polymer latex composition as defined above.
According to a preferred embodiment the adhesive is for use as a fast setting adhesive for paper and packaging, said adhesive comprising a vinyl acetate homopolymer latex composition.
According to another preferred embodiment the adhesive is for use as a wood glue and comprises a vinyl acetate based polymer latex composition of a copolymer of vinyl acetate/N-methylol acrylamide, optionally further including versatic acid vinyl ester having 9 to 13 carbon atoms from the carbonic acid group. The chain transfer agent is preferably mercapto propionic acid.